1. Technical Field
The present disclosure relates to a lithium ion secondary battery.
2. Related Art
Lithium ion secondary batteries are lighter in weight and higher in capacity than nickel-cadmium batteries, nickel-metal hydride batteries, and the like. The lithium ion secondary battery has been therefore widely used as a power source for a mobile electronic appliance. Further, as the mobile electronic appliances decrease in size and increase in functionality in recent years, the lithium ion secondary battery has been expected to have further higher capacity. Not just for the mobile electronic appliances, furthermore, the high-capacity lithium ion secondary battery has been a strong candidate as a power source to be mounted on a hybrid vehicle, an electric vehicle, or the like.
The increase in power storage amount per unit mass of a positive electrode and a negative electrode leads to the increase in energy density of a battery. As a positive electrode material that can increase the power storage amount, a so-called solid solution-based positive electrode has been examined. Above all, a layered oxide containing lithium is expected as a candidate of the positive electrode material with high capacity. The layered oxide containing lithium has an excellent cycle characteristic and can exhibit an electric capacity more than 200 mAh/g depending on the charging/discharging condition.
A positive electrode including the layered oxide containing lithium has high discharge capacity. However, in this positive electrode, oxygen is easily released from a positive electrode active material due to temperature rise when the deintercalation of Li out of the active material is caused by overcharging. This accelerates the oxidation reaction, which causes a problem that the temperature in the battery rises excessively.
For solving the above problem, for example, a battery according to JP-A-10-233237 includes an endothermic layer containing magnesium carbonate outside a positive electrode active material layer. In this configuration, when the battery is short-circuited, endothermic reaction occurs in which carbon dioxide is released, so that the battery is cooled.
Moreover, a battery according to JP-A-2004-146296 has lithium carbonate introduced inside the battery. Thus, lithium carbonate reacts with silicon oxide in a negative electrode in a process of heating due to short-circuiting current during the internal short-circuiting of the battery. As a result, carbon dioxide (CO2) and lithium silicate are generated. The generated CO2 fills the inside of the battery to reduce the oxygen partial pressure. Thus, excessive heat oxidation reaction is suppressed. As a result, the temperature rise of the battery is suppressed.